Counting devices



Dec. 22, 1959 E. Roo-r 3RD 2,918,215

COUNTING DEVICES Original Filed Aug. 13, 1947 5 Sheets-Sheet 1 IN V ENTOR.

ATTORNEYS V.. B mv\/\/ Q 9 Nm mm om Sv N mm wv mm a w @n i N w M x @Nvom NON mm w vm Dec. 22, 1959 Original Filed Aug. 13, 1947 E. ROOT 3RDCOUNTING DEVICES 5 Sheets-Sheet 2 ATTORNEYS Dec. 22, 1959 E. ROOT 3RD2,918,215

COUNTING DEVICES Original Filed Aug. 13. 1947 5 Sheets-Sheet 5 l ENTOR.

Dec. 22, 1959 E. Rom 3RD 2,918,215

COUNTING DEVICES original Filed Aug. 15, 194'/ 5 sheets-sheet 4 FRINGES0 'I 2 3 4 5 MoTloN l PHASEI MULTIPLER E/ E/I MUPLAASEE PHASE IA TR|GGERPHAsElI B j TRlGGER y sTAGEl C PHAsEI STAGE D l l I l l PHASElI STAGE 2E PHASEI [N VEN TOR.

ATTORNEYS E. ROOT SRD Dec. 22, 1959 COUNTING DEVICES 5 Sheets-Sheet 5Original Filed Aug. 13, 194'? INVENTOR.

AT TORNEYS FIG.9

United States Patent 'O COUNTING DEVICES Elihu Root 3rd, Springfield,Vt.

Original application August13, 1947,*Serial No. 768,300,

now Patent No. 2,604,004, dated July 22, 1952. Divided and thisapplication June 21, .1952, Serial Nm 294,811

7 Claims. (Cl. 23S- 92) The present invention relates to countingdevices, and more particularly to devices for making additive andsubtractive counts. This application is adivision ref mycopendingapplication, Serial No. 768,300, iiledAugust 13, 1947, now Patent No.2,604,004 granted Julyf22, 1952.

The principal object of the yinvention is 4to provide a reversiblecounter, i.e., a counter capable ofat'lding or subtracting, in which thepossibility of false counts due to vibration or to uncertainties orambiguities yat transition points is avoided. l

The purposes with which the invention arerconcerned will be appreciatedupon consideration of the 'use of the counter in combination withmeasuring apparatus iwhereby an article is measured bycouuting'a.successionofy interference fringes, as described in myabove-mentioned application. The counter is operated in what may betermed a substantially continuous fashion, as distinguished from thecounting of impulses. Whilereversiblecounters for continuous phenomenahave been proposed,.`they have, so far as I am aware, `sufferedrfrollafdefect in the nature of backlash inthat reversalsfwhichloccur in theneighborhood of a transition pointmaygiverise to miscounts. I

According to the present invention ,thecountingysystem is free ofbacklash and is subject to continuous reversible control in theneighborhoodpf.,ajtjausition point, whereby a correct net count ,isobtained, notwithstanding oscillations, whether dueto noiseA Y V wise,that may occur in-the system. A'lfhehi embodied in both mechanical' andeleetronic will hereinafter appear.

Further features of the inventionconsist ,ofzcertlalin novel features ofconstruction, eomhinatiosnd arrangements of parts, hereinafter,described Aand particularly defined in the claims.

In the accompanying drawings Fig. 1isa Idiagram of ing certainconventional elements but with special features peculiar tothe presentinvention. The light source the preferred form of measuring and optical,apparatus;

Fig. 2 is a view of the field of illumination due tothe formation ofinterference effects by one of the reflecting mirrors; Fig. 3 is adiagram of the electrical connections;

Fig. 4 is a diagram of one form vof countingv apparatus;

and Fig. 5 is a graph illustrating operation of thecouiiter; Fig. 6 is adiagram of a portion lof;tl`1ecounter of Fig. 4; Fig. 7 is a diagramshovs/ingtliecounting-.sequence; Fig. 8 is a diagram illustratingtheprinciple of the counter; and Fig.,9 isa diagram` of ,ther-motor*control circuit.

The counting system of-the present invention is described as embodied ina system forprecise Vvmeasurement by the counting of changes ofillumination vdue to interference phenomena. To aid inanrun'derstandingof the operation of the countingsystem thepreferred form of measuringapparatus, as described inmy.above mentioned application, will.be`described,l,1ere

The apparatus shown in the drawing ofFig.`1, coxn prises a measuringhead; indicated generally,at'.v6,an

optical device camprising. an .intensiever-grinding? Patented Dec. 22,1959 `generally at 8, a source of light for the optical system indicatedat 10, and photo-sensitive means indicated at `1,2 responsive to thechanges of illumination from light y'to .dark and dark to light. Thesechanges are referred 'to herein by the technical term fringes althoughit will `be understood that the eld does not present the succession oflight and dark rings or bands frequently seen in interference phenomena;in other words, the (term fringe as here used simply means a halfWavelength.l The invention further comprises counting means to .be laterdescribed in detail.

v Referring first to the measuring device 6, this comprises a base 14 onwhich is mounted a carriage 16 :adapted to be driven by a motor 18 whichis connected to thevcarriage through suitable gearing 20 and a screwA22. The base carries an anvil 24 and the carriage 16 carries'a head 26adapted to be moved toward or from an anvil 24. As shown in the drawingthe head 26 is resiliently mounted on the carriage by means of leafsprings orreeds 28 and 30. Posts 31 and 32 extend upwardly frornvthecarriage and form a yoke embracing a projection 34 depending from themeasuring head. The yoke is provided with screws 35 which may beadjusted to give any desired lost motion between the carriage and head.

opened. lf apiece of work 25 is placed adjacent to the Ianvil 2 4 andthe motor is operated, the head will move tintili'it engages the workwhereupon the contacts are opened because of the flexure of the springs28 and 30. Further details of the head and a servo control thereforrshown in Fig. 9 will be described later.

The'head is provided at its outer end with an accurate `at reflectingsurface indicated at 42 which forms La, part of the optical system.

The optical system comprises an interferometer involvv10 is preferablyavtube capable of emitting sharp spectralllines,vsuch as a kryptondischarge tube. The light from the scource is directed through adiaphragm 43, a

"'collimating lens 44 and'prism 46 through a halfreflectstrip or band 53is provided in the mirror 52, this strip being likewise a retlectingsurface and spaced from the iside portions 54 of the mirror by adistance equal to oney eighth of a wavelength of the light emitted fromthe source 10. Its formation may be accomplished by placing a mask overthe central portion of the mirror While the metal is deposited on theremainder of the mirror. Or the deposit may be formed on the portion 54while the rest'of the mirror is masked in which case the band will Ibe'exhibitedas a raised portion instead of a depression in the surface. Itwill be understood that any arrangement may be used which will providetwo adjacent reflecting surfaces differing in height by an eighthwavelength ,(oran. add multiple thereof). In any case it is desirable tohave one phase (here represented by the strip 53) embraced between twosymmetrical outer portions 54,

which constitute the second phase. As will be shown later,` the opticaloutputs of the two phases are fed to photo-'sensitive devices togenerate a two-phase electrical be lost. It can be shown, however, thatwith the mirrorA 52 having symmetrical outer portions, the contributionsfrom the outer portions will add vectorially in such a way that a fairlylarge misadjustment of the mirror produces no substantial departure fromthe quadrature relation. Any arrangement may be used which` splits thearea representing one or both phases, so that each phase is"symmetrically disposed with respect to the center of the mirror as awhole. The mirror 52 is mounted in a suitable frame 56 arranged foraccurate angular adjustment.

The light is focused by the lens 50 into the plane of a diaphragm 58having an aperture to pass only one of the spectral lines, the severallines having been dispersed by the prism 46. If the light passingthrough diaphragm 58 were intercepted by a screen in the position ofmirror 60, the pattern of light would appear as in Fig. 2. For.. oneposition of the measuring head the central band would constitute a dark"fringe due to the interference phenomenon involving light reliectedfrom the central strip 53, while the outer portions of the mirror 54show mean illumination. This is for one position of the measuring headmirror 42. Motion of the mirror 42 by a quarter wavelength (so that thepath from 48 to 42 and back again is changed by a half wavelength)results in changing the image of Fig. 2 to a light center band with theoutside portions again at mean intensity.

The light from the band 53 is reflected by the mirror 60 to thesensitive element of photocell 62. The light reflected from the outsideareas of the mirror 52 passes the mirror 60-and falls on a secondphotocell 64. The cell l 64 constitutes one phase of the photosensitivesystem,

herein called phase I, while the cell 62 constitutes a second phase(phase II). The photocells. 62 and 64 are included in electronmultipliers 66 and 68 of conventional form.

Upon motion of the head 26 alternate dark and light interference fringesare formed at the surface of the mirror 48. Considering only thereection from the center surface 53, the light falling on the photocell62 will -change from minimum to maximum and back to minimum for eachhalf wavelength of motion of the head 26.'`

The variation of light falling on the tube 62 is substantiallysinusoidal. Y

Because of the spacing of an eighth Wavelength between the surfaces 53and 54, the light variations projected on the cell 64 will likewise besubject to a sinusoidal variation but with a phase difference of aquarter wavelength (90 degrees) from the variations of intensity on thetube 62. The phase of the variations in the tube 62 either leads or lagsthat ofthe tube 64, depending on the direction of movement of the head26. Thus whenthe head is moved in one direction, there is a 90 phaselead and when the head is moved in the opposite direction, there is a 90phase lag between the two tubes. This difference between lead and lag isutilized to dictate whether the count of fringes at any particular timeis additive or subtractive.

As shown in Fig. 3 the outputs of the multipliers 62 and 64 are fed tosuitable amplifying, recording and registering circuits. The output ofthe phase I, namely, that from the tube 64, is fed to the plates in twoquadrants of cathode ray tube 74. The output of phase II is fed to theother two quadrants of the tube 74. It will be apparent that by theapplication to the tube 74 of voltages which differ in phase, both intime and space, there will be a n made.

of the head is made sufficiently slow, the rotations of the spot both inthe forward and reverse direction may be counted by an operator.However, since there are about 100,000 wavelengths per inch, such acounting procedure would be too slow and laborious except in themeasurement of extremely small distances. Accordingly, the tube 74 ispreferably not used as a counter but is provided to give aninterpolation for fractional wavelengths. The counting of integralfringes is then preferably accomplished by electronic or mechanicalmeans. By noting that each revolution corresponds to one-halfwavelength, the ultimate measurement may be determined to an accuracy interms of a small fraction of a wavelength.

In order to effect automatic counting, the outputs of the tubes 64 and62 are fed to suitable amplifiers or trigger circuits indicatedrespectively at 76 and 78 whereby the sine-wave outputs may be convertedto rectangular waves in a well known manner. These rectangular waves areseparated in phase by 90, either leading or lagging depending ondirection of motion ofthe measuring head. The outputs of the ldevices 76and 78 are then fed to a counter by which additive and subtractivecounts may be The preferred form of counter is an electronic counter forseveral stages wherein rapid counting is essential, after whichmechanical stages may be preferably used. l Actually, a mechanicalcounter may be used throughout if operation at low speed is acceptable.Ac-

f cordingly, for simplicity only, a mechanical form of counter is firstdescribed.

The output of the tube 76 for phase I is fed to the grid of va triode80. A second triode 82 is also energized from the output of 76 through aphase inverter 84. Thus the anode circuits of triodes S0 and 82 forphase I have anode circuit currents which are 180 out of phase with eachother.

The output of the trigger 78 for phase II is fed'ln identical fashion totriodes 86 and 88, the latter being energized through a phase inverter89. The anode cirto a coil 92. The coils are mounted on a core 94 havingpoles between which is placed an armature 96. The

armature is adapted to be attracted to one pole or the other dependingupon the energization of coil 90 or coil 92. Similarly for phase II theoutput of the triodes 86 and 88 is connected to coils 98 and 100 on acore 102.

An armature 104 is adapted to be attracted to one or the other of thepoles.

The armatures 96 and 104 are connected to operate "a multi-tooth starwheel 106 which s mounted for rotation in any suitable manner. A lengthof wire 108 is satisfactory for the short length here required. The

rotating trace on the screen of the tube. This trace will rotate onerevolution for the motion of the head 26 corresponding to one-halfwavelength of light. If the motion la compound motion. up vof thetangential (up and down) component due to the'motion of fthe wire 108together withk a sidewise `wire is provided with two pins 110 and 112which are selectively adapted to enter between selective teeth of thewheel 106. Thus, in the position shown, the pin 110 is between two teethwhile the pin 112 is clear of the teeth.

This is the condition existing when the armature 96 is attracted to theupper pole. Upon energization of the llower'pole through the coil 92 thepin 110 is moved downwardly.

A wire 114 is connected between the pin 112 and a Asuitable fixed point.Another wire 116 is connected from .the armature 104 to the pin 112.

It will be seen that the pin 112 is primarily a detent member andsubstantially constrained to undergo a simple radial (left and right)motion by reason of the connection of the wire 114. 0n the 'other hand,the pin 110 is free to undergo This compound motion is made ,5 componentdue to the lateral motion of the wire 116. T he compound motion ispermitted4 because ofthe exibility of the wire 108 between the pin 110and the pin 112. Since the coils of the magnet 102 are energized 90 outof time phase with the coils of magnet 94 and since the wires 108 and116 are phased'in space by 90, the pin 110 will undergo a rotary motion.The direction of the rotary motion of the pin 110 depends on whether thetime energization of the coils 98 and 100 is in a leading or laggingrelation to the energization of the coils of the magnet 94. Thus, for aforward motion of the measuring carriage 26, the pin 110 Will undergo,rsay, a clockwise motion, while a reverse motion of the carriage willcause the pin 110 to undergo a counter- -clockwise motion. The range ofmotion of the pin 110 is so related to the spacing of the teeth that onetooth is advanced for each revolution of the pin. It will be observedthat the pin 110 engages with an intertooth space at the left of thewheel just as pin 112 is being retracted from the inter-tooth space atthe right of the Wheel so that the motion is permitted. When the pin 110retracts from engagement, the pin 112 enters an intertooth space andthis serves as a detent to prevent overrotation.

Because of the lightness of the parts, it is possible to count fasterthan with conventional mechanical counters. The wheel 106 is advancedone tooth for each complete rotation of the trace on the cathode raytube 74. Hence, the position of the Wheel 106 indicates the net numbercount on any succession of motion of the carriage 26. The position ofthe spot on the screen of the tube 74 may be used to interpolate formore precise indications.

The trigger circuits 76 and 78 are not essential, and if-desiredthesinusoidal outputs of the multipliers may be amplified linearly andfedto the counter. Squarewave actuation is preferred, however, inorderto reduce the variations to substantially constant intensity. It will beunderstood by those skilled in this art that the fringes are of greatestclarity (i.e. there is a maximum difference between light and dark) whenthe lengths of the two paths of light are equal. As the mirror 42 movesaway from the position of greatest clarity, the fringes become weaker.The extent to which the mirror 42 may be moved before the fringes becomeindistinct depends partly on time of vibration of the atoms of the lightsource. Over the useful range of movement the outputs of the multipliersvary considerably and hence the trace on the cathode ray tube will be acircle of changing diameter. This is not objectionable so far as `thecathode-ray tube is concerned, because the amplifiers for thecathode-ray tube are so biased that if the light falling on bothmultiplier tubes t spect to the center of the screen continues to giveanaccurate measurement of fractional fringes regardless of thedistinctness of the fringes. However, the input `to the counter shouldbe of nearly constant intensity, in order that the range of motion ofthe pin 112 shall be sensibly uniform. The same requirement applies toan electronic counter. This is best accomplished by squaring the outputsof the amplifiers 86 and 88 in any suitable manner, here indicated bythe use of trigger circuits.

It will be observed that the input to the counter is different from thatfrequentlyemployed in counting devices, since the counter may be said tobe subjected to Icontinuous or non-impulsive excitation. This excitationis preferable whether a mechanica-l or electronic counter is used.

An electronic counter operating on a similar principle iis shown in Fig.4. This figure shows aA circuit for a binary electronic counter stage.As manyv stages may 'be used as are necessary for counting toany desirednumber. Inthe subsequent description of this' circuit, in

order to avoid complicated cross connections on lthe draw@ ing, some of4the connections are' given symbolically. Thus, all points bearing-thesame designation are connected to a common point. For example, allcircles marked A are connected together, all circles marked B areconnected together, all circles marked A' are connected together and soon.

To introduce the electronic counter, the system of Fig. 3is cut off atthe dot and dash line, and the circuit of Fig. 4 is added to the lefthand portion of Fig. 3 by joining correspondingly lettered connectionsat the cut. The circuit in Fig. 4 will now be known as stage 1. Thecircuits for succeeding stages follow the pattern of Fig. 4 exactly.

The single stage shown in Fig. 4 includes four triodes. These aregrouped in two trigger pairs, which are of conventional constructionexcept for the network of connections controlling their 'operationsTriodes 122 and v124 form one trigger pair. Triodes 126 and 128 form theother pair. It will be observed that al1 connections are direct asopposed to capacitative. The inputs which drive the stage are points A,A', driven by phase I, and B, B', driven by phase II. It will beremembered that by reason of the phase inversion in tubes 84 and 89 thepotential of A is always high While A' is low and A is low While A' ishigh, likewise for B and B.

Consider the trigger pair comprising triodes 122, 124. Each anode isconnected to a source of positive potential through resistances 129,130.v The anode of each tube is connected to the grid ofthe companiontube through a resistor. These resistors for phase I are shown at 132and 134.

The anodes of the tubes 1212 and 124 are connected to the outputterminals designated C and C'. These points are shown conected to theright hand edge of Fig. 4 indicating that they form part of the inputcircuit ofstage 2 in a manner exactly analogous to the way in whichterminals A, A', the output of phase I form part ofthe input circuit ofstage 1.

The grid of tube '122 is connected through a resistor `136 to point Bwhich, as heretofore noted, forms one of the terminals for the input ofphase Il. Similarly, the grid of tube 124 is connected through aresistor 138 with the point B. The grid of each tube is also connectedto a control circuit, which for the tube 122, is indicated generally bythe character The control circuit 140 comprises a pair of parallelresistors 142 and 144, the latter being connected to terminal A and theformer to terminal D which is one of the output terminals of phase II.This control circuit also includes another pair of resistors 146,` 148,the former being connected to point D and the latter to A. All of thesefour resistors are connected at the bottom to the grid of tube 122except, it will be noted, that resistors 142 and 144 are connectedthrough a rectifier 150,` while the other two resistors are connectedthrough a rectifier 152 to the cornmon connection 154 which leads to thegrid. The rectiers rnay be of any simple construction, preferablyselenium or germanium crystals. The object is to prevent one set ofresistors from neutralizing the effect of the other, as will laterappear.

The control circuit also includes resistors 1'56 and '1'58 which areconnected respectively to the separate resistor pairs previouslydescribed. The resistors 156 and 158 are together connected to a sourceof negative potential.

A similar control circuit 160 is connected to the grid of tube 124. Itis identical with the control circuit 140 except that its terminals areconnected respectively to terminals D', A', and D, A.

The pair 122, 124 constitutes a trigger pair which, by itself, operatesin conventional trigger fashion. That is to say, either the tube 122 hasits anode circuit conducting and the tube 124 non-conducting, orvice-versa. Point C is at high potential when the anode circuit of tube122 is' non-conducting, while at the same time point C' is at lowpotential because the anode circuit of tube 124 is conducting. Uponoccurrence of a situation in which conduction through 124 is cut off,point C' goes immediately to high potential. The transfer of potentialthrough resistor 132 from the plate of 124 to the grid of 122 thencauses tube 122 to assume the conducting condition so that point C thengoes to low potential. Stable equilibrium can exist only when one tubeof the pair is conducting and the other is non-conducting. Thus, thereare two possible conditions of stable equilibrium, and there is always ashift from one to the other whenever conduction is cut ol in thepreviously conducting tube.

The manner in which the trigger circuit is operated is described asfollows: Assume that 122 is in the low potential condition, namely thecondition in which its anode circuit is conducting. Its grid is nowpositive because of the positive potential transferred from the anode oftube 124 through resistor 132. A connection 161 is made from a source ofnegative potential to the grid of tube 122 and also a connection is madefrom the same negative source through resistor 163 to the grid of tube124. The various potentials are such that when B goes to a low potentialcondition and D' and A both go to a low potential condition at the sametime, the elect will be to make the grid of 122 suciently negative sothat it can no longer conduct. These are not the only conditions formaking tube 122 non-conducting but are cited as a specific example.Whenever tube 122 becomes nonconducting, the potential of point Cimmediately rises and this converts tube 124 to the conductingcondition.

The circuit diagram for potentials on the grid of tube 122 is given inFig. 6. Resistors 161 and 156 are connected to a negative potentialsource. When the triode 122 is conducting (C at low potential), terminalC is at high potential. The potentials at A, B and D and the values ofthe resistors are such that if any one or more of the points A, B, D'are at high potential, the grid of 122 will not be sufficiently negativeto cut ol conduction. However, when all those points are at lowpotential, the grid goes negative sufficiently to cut oi conduction, Cgoes to hi@ potential, and conduction is transferred to the other triodeof the pair.

Conduction in 12'2 can likewise be cut oi if terminals B, A and D areall at low potential at the same time. The purpose of the rectiiiers 150and 152 will now appear. These rectiiiers isolate any net positivepotential of A, D' or A', D from the grid; if they were not present, a.low potential at A, D' might be neutralized by a high potential at A',D.

Before further describing the operation of the system,

it should be noted that the second trigger pair 126, 128

is set up in substantially identical fashion to that rst described. Theanodes are indicated by the terminals D and D which constitute theoutput terminals of the trigger pair. The grids are connected throughresistors 136' and 138' to terminal B. The grids of the tubes areconnected to control circuits 162 and 164 which are identical with thecircuit 140. The control circuit 162 is connected to terminals C', A, C,A', while the control circuit 164 is connected to terminals C', A', C,A.

The operation of the electronic counter stage may best be described byreferring to Fig. 5, which shows the space variation of potentials atvarious points in the system for various positions of the measuring headwhich are indicated in terms of fringes at the top of Fig. 5. For themoment let us assume that the measuring head is moving uniformly fromleft to right. This will allow us to consider the potential variationsin terms of time. The uppermost curve shows the output of the multiplierof phase I. This is a sine wave. The second curve is also a sine wavewhich represents the output of phase Il. This second curve is 90 out ofphase with the upper curve. Upon passage through the trigger circuit 76,the phase l output is converted to the square wave shown in the thirdcurve. This represents the potential of point A. The output of phase IItrigger circuit is shown in the curve immediately below and thisrepresents the potential of point B. The potentials of points A' and B'are not shown on this drawing and are unnecessary because when A is athigh-potential, A is at low-potential, as above described. The nextcurve shows the variations of potentials for points C and D. It ispossible to show how the variations in the curve for potentials at C andD depend upon the preceding square Wave curve for A and B. Take the timerepresented by the iirst vertical dot-and-dash line represented by t1.At that time, points A, B and D are at high potential and C is at lowpotential. These conditions are mutually consistent. After a short time(indicated at t2) B goes to low potential and correspondingly, B Igoesto a high potential condition. This does not make any immediate changein either C or D, because the potential on the grid of 122 has not beenreduced suiciently to cut oi'f anode conduction for that tube. However,after a short time, represented now by the dot-and-dash line t3, point Adrops to low potential. Points A, B and D' are now all at low potential.Therefore, as shown by Fig. 6, the potential on the grid of the tube 122becomes sufciently negative so that conduction in tube 122 is cut oi. Atthis time point C rises to high potential and correspondingly, point Cdrops to low potential. Point D continues in its high potentialcondition as indicated by the curve.

A complete analysis of this type shows that the conditions forconversion of any of the anode terminals from low potential (conducting)to high potential (non conducting) condition are as follows:

Anode: Terminals at low potential C B, A, D or B, A', D C B, A, D or B,A', D' D B', A, C' or B', A', C D' B', A, C or B', A', C

A complete cycle of operations for stage 1 is illustrated in Fig. 7.Here the four terminals C, C', D, D' are indicated for the times t1, t3,t5, t7 and t9. A light circle represents a low-potential(anode-conducting) condition while a hatched circle represents ahigh-potential or non-conducting condition. Starting with t1, C and D'are low and C' and D are high. There is no change at At t3, C is cut offand C' goes to low potential. At t5 there is an interchange between Dand D', at t7 another interchange between C and C', and at i9 anotherinterchange between D and D which carries the pattern back to theoriginal condition.

The foregoing is an example of counting in one direction which may beconsidered forward" counting. In that case, phase Il leads phase I.Assume that irnfmediately after t9 the carriage starts to move backward.The Variations in conditions are now found by following the curves inFig. 5 from right to left. The various states which appeared previouslynow reappear in inverse order. The pattern of Fig. 7 is followed outback- Ward, that is, at some times tu, 213, t15 and tu, the conditionsgo progressively through those shown for t7, t5, t3 and l1. This isbackward or subtractive counting. It will be noted that in this casephase I may be said to lead phase II.

In adding stage 2, the respective output terminals C, `C and D, D'become also the input terminals of stage 2, which is identical tostage 1. The corresponding output terminals of stage 2 can then belabeled respectively E, E and F, F'. The two lower curves in Fig. 5represent the variation in potential of points E, F. It will be seenthat the pattern of potential changes for E, F is identical with thepatterns for A, B and C, D except ythat the time base is again doubled.Furthermore, it -will be seen that the relation of changes in E, F tochanges in C, D is identical to the relation of changes in C, D tochanges in A, B. Each stage may be conasians sidered as the prime driverof its succeeding stage in the sense Vthat it is necessary to considerchanges only in the former in order to determine the response of thelatter. Stages may be cascaded as desired, and a mechanical counter ofthe type shown in Fig. 3 may be used at the end if desired.

Reading of the contents of the counter is accomplished by consideringthe state of one trigger pair in each stage ,to represent the value of abinary digit corresponding to that stage. Thus, referring to Fig. 5, thepotentials, at a given point, of E, C, A considered as representing thevalue 1 when high, 0, when low, will indicate in binary form the numberof half fringes between the left hand edge of the iigure and the givenpoint.

'-It will be observed that since there are two trigger pairs or elementsin each stage and each element can exist stably in either of two statesthere are four possible states for the stage as a whole. However, eachStage indicates the value of only one binary digit and only one of theelements of the stage is required for this indication. There is thus anapparent redundancy of states and elements. Nevertheless, the fullnumber of states and elements is required in order to maintaincontinuous reversible control through the chain of stages. This type ofcontrol will now be contrasted with the rusual impulsive control andreasons for preferring it to impulsive control will be described.

` In the discussion which follows use will be made of the termtransition point. Referring to Fig. 5, any position where a trigger pairchanges state is a transition point. In particular the transition pointswhere C changes state may be termed count points since these points markthe registration of whole fringes. A transi' tion point is the samepoint regardless of the direction in which it is traversed. Referring toFig. 8 which represents a view of cathode ray tube 74 it will be seenthat a count point is marked bythe passage of the spot through line X.Other details of Fig. 8 will be discussed later.

When a transition occurs in the counter some of the elements changestate while others remain in the same state. In general the change ofstate of any element, which we will term a controlled element, isbrought about directly by the change of state of an element in thepreceding stage, which we will call a controlling element. At the sametime certain elements which we will call holding elements, which do notchange state at this particular transition, exercise a latent control onthe controlled element in .the sense that their states determine theexistence and polarity of the control exercised by the controllingelement.

The fact which especially characterizes this counter, is that intheregion of any given transition point, the control existing between anycontrolled element and its controlling element is continuous andreversible in the sense that in the region immediately on either side ofthis particular transition point the existing steady state of thecontrolled element is uniquely determined by the existing steady stateof the controlling element. This means thatx in the event of oscillationabout a single transition point it is not necessary that the transitionpoint be crossed decisively or that time be allowed at each crossing forthe state changes initiated by that crossing to proceed to completion,in order to prevent a miscount.

Furthermore, the only requirement on the response speed of an element inany stage is that after passing a transition point involving thatelement, that element should have substantially attained its appropriatesteady state before a different transition point involving that stage isreached. Since the separation between transition points involving agiven stage becomes greater the higher the stage, the response speedrequirement becomes correspondingly less.

' In contrast .to the situation described above, I am aware that itispossible to effect reversible counting with an ipnpulsive counterwherein a forward pulse is registered in passing the count point inaforward direction, and a reverse pulse is registered in passing thecount point in the reverse direction. Such a counter can be built havingonly one element or trigger pair per stage. It suffers from thedisadvantage that in the case of an excursion across a count point andreturn, time must be allowed for registration of both a forward andreverse pulse. 'If the two crossings occur too close together, one ofthe pulses may not be registered properly and a miscount will occursince the final steady state of the controlled element is not uniquelydetermined by the state of the controlling element, but requires also ahistory of correct pulse registration.

In order to prevent such too close crossings of a count point which mayoccur either as a result of high frequency noise in the primary signalor simply as the result of a slow excursion which passes the count pointby only a minute distance, it is necessary to introduce backlash. Thisis illustrated in Fig. 8. Assuming that the spot moves clockwise inforward counting, then for the impulsive counter the count point in theforward direction is made to occur when the spot crosses line Z, whilethe count point in the reverse direction is made to occur when the spotcrosses line Y. The amount of backlash is indicated by the distancebetween Y and Z and should in general be greater than the oscillationsdue to noise. When the interferometer is set for greatest sensitivity,that is, when both optical paths are of nearly equal length, the tubewill show a trace represented by circle 166, but when the paths differmarkedly, the sensitivity goes down and the trace becomes somewhat asshown at 168. Obviously the smallest trace which can be counted has adiameter greater than the backlash. Therefore the use of a continuouscontrol counter, which does not require' backlash, will increase thepermissible counting range. It should also be pointed out that,especially when the` noise amplitude is reduced by filtering, the .tracesize isv reduced as the counting speed increases, and the continuouscontrol counter with its lack of backlash can be used at higher speeds.

The non-impulsive mechanical counter previously described shares withthe preferred electronic counter the qualtities of continuous reversiblecontrol and of requiring no backlash for reliable operation.

The counters herein described, while exceptionally useful in combinationwith the measuring and optical devices of the present invention, are notlimited thereto but are of general application.

Mention has been made of the motor circuit by which motion of the head26 toward and from the work may be controlled. This is described ingreater detail in connection with Fig. 9 which is a wiring diagram forthe motor circuit. The motor 18 is energized through two relays and 172.38 previously mentioned. The switch 38 has two contacts 174 connected toone line and mounted on but insulated from the carriage. For forwardmotion of the carriage, one of the contacts 174 is closed on a contact176, connected through contacts 178 of a key-operated manual switch 180to the winding of relay 172, which in turn is connected to the otherline.

The relays 170, 172 have armatures 182, 184 respectively, normally heldagainst upper fixed contacts by suitable springs and adapted to beattracted toward the lower contacts upon relay energization. Thepositive line is connected to both upper contacts by wires 188 and 190,while the negative line is connected to both lower contacts by wires 192and 194. The motor armature is connected to the relay armatures 184,186.

When the switch contacts 174, 176 are closed and the contacts 178 areclosed by the key, relay 172 is ener-v gized. The motor circuit is thenclosed from the plus terminal through the wires 188, 190, the relayarmature 182, the motor 18, the relay armature 184 and the wire Thecarriage 32 carries the switch 1'1 192 to the negative line. The motorthen operates in the direction to move the carriage toward the work.When the anvil engages the work, the contacts 38 are opened, therebydeenergizing the relay 172 and causing its armature 184 to close on theupper contact to shortcircuit the motor and bring it quickly to a stop.The carriage may now be backed away from the work. This may be donemanually by turning the key of the switch 180 to close contacts 196which are in series with the relay 170. Energization of the relay 170causes operation of. the motor, but with reversed armature connections,whereby the motor operates in the reverse direction.

It is to be observed that the forward motion of the carriage can beeffected only when the switch 173 is closed, this switch being in serieswith the switch contacts 174, 176. Backing off motion of the carriagecan, however be effected either by closure of the switch contacts 196 orby an additional contact 198 on the carriage. It will be noted that byreason of the connection 200, the contacts 174, 19S are in parallel withthe contacts 196, whereby closure of either set operates the motor inthe back-off direction. The principal purpose of the contact 198 is tocause a back-off motion of the carriage when a piece of work is forcedinto position between the measuring anvils. In that case the motor backsthe carriage away until all contacts of the switch 38 are open.

Referring -to Fig. l, the head 26 is normally urged forward by a spring202 connected with an adjusting screw 204 in a projection extendingupwardly from the post 32. In order that the measuring contact pressuremay be substantially independent of the position of the head withrespect to the carriage, the variation in force exerted by spring 202and reeds 28, 3l) may be counteracted by av negative spring. 'I'henegative spring is shown at 286 and comprises a flat spring memberreceived in opposed notches of the carriage and the post 31, the springbeing bent so that it exerts a straight upward pressure on the head inneutral position. When the head is deflected to either side of neutral,the force of spring 2&6 has a lateral component which, over a shortrange, is substantially proportional to the deflection and in adirection which tends to increase the delection. This force balances thenet force differential of the combination 28, 3i), 2M which is alsoproportional to the deflection but in the normal direction which tendsto reducethe deflection.

In operation for absolute measure of a work piece such as that indicatedat 25 in Fig. 1, the head 26 is brought up into contact with the Ianvil24. This is the zero position. The head is then backed away suflicientlyto allow for introduction of the work piece 25. After introduction ofthe work piece, the head is moved in the opposite direction until itcontacts the piece 2S. The counts made in retracting the head may beconsidered as in the positive direction and the counts made in movingthe head toward the work piece may be considered as being in thenegative direction. Or conversely, the head may first be moved up to thework piece, then retracted so that the work piece can be withdrawn andthen finally moved up into contact with the anvil 24. ln any case, thedevice counts both forward and back whereby the actual measure of thework piece is indicated. In any case, where measurements againstsecondary standards are acceptable, the time of measurement may beshortened by merely moving the head 26 suticiently to accommodatesuccessively the work piece and a standard gage member.

An absolute measurement may be made, in one. step, o-ver the entirerange of distinct fringes which in the case of krypton 5870A and with ausefully large diameter for aperture 53, is at least twoinches. Forlonger measurements, the carriage 16 may be clamped inv any suitablemanner. The head 26 rests against the backstop to carriage 16. Then thecounter is shut olf and the;

entire interferometer (including the light source, prism,V

etc.) is moved relative to the mirror 42 to establishy a. new startingpoint. carriage is unclamped, and the measurement proceeds as before.Any vibration introduced during the clamping or unclamping of thecarriage is fed into the counter and is thus automatically accountedfor, without error. It is only necessary to assure that the action ofmoving the interferometer parts does not move the carriage and` viceversa. By this stepping means, an absolute measure of substantially anylength may be obtained. InI making such a measurement, it is desirablealways to move the interferometer body an approximately integral numberof fringes as indicated by the cathode ray tube interpolation pattern(which is not turned olf) and tokeep track of the cumulative effect ofany small departures from this integral fringe motion.

Having thus described my invention, I claim:

1. In a reversible digital counter stage, the combination of means forgenerating two non-impulsive inputs. which are phase-displaced relativeto each other, each input being capable of rapid transitions between twovalues, trigger means having a plurality of stable statesA andresponsive to one input to generate a non-impulsive output, additionaltrigger means responsive to the other yinput to generate a non-impulsiveoutput, and coupling. means between the outputs and inputs to apply tosaid trigger means combined signals to effect a transition of onetrigger means depending on the state of the severalf inputs and outputs.

2. In a reversible digital counter stage, the combination of means forgenerating two pairs of inputs, the inputs of each pair being ofopposite phase and the two pairs being phase-displaced relative to eachother, a first and a second trigger pair each having two Stable statesand arranged to produce two phase-displaced pairs of outputs, and meansfor applying to the separate trigger pairs combined inputs derived frompotentials of saidy input pairs and said output pairs.

3. In a reversible digital counter stage, the combina tion of means forgenerating a pair of oppositely phased inputs designated A, A' and asecond pair of oppositely` phased linputs design-ated B, B; the saidpairs beingA phase-displaced relative to each other; a first and asecond trigger pair each having two stable states and arranged toproduce output pairs C, C' and D, D', respectively, similar to the inputpairs but of double period;` means for applying input B to the firsttrigger pair and input B' to the second trigger pair, means foradditional ly applying to the first trigger pair inputsinvolving-combinations of A, A and D, D', means for additionallyapplying to the second trigger pair inputs involving combinations of A,A' yand C, C', and means for causing a change of state of a trigger pairupon occurrence of predetermined relations among its several inputs.

4. In a reversible digital counter stage, the combination of means forgenerating a pair of oppositely phased inputs designated A, A' and asecond pair of oppositely. phased inputs designated B, B; the said pairsbeing phasedisplaced relative to each other; a first and a second trigfger pair each having two stable states and arrangedl to produce outputpairs C, C' and C, D', respectively, similar to the input pairs but ofdouble period; means for applying to the rst triggerpair combined inputsderived from B and combinations A, A' and D, D'; andv means lforapplying to the second trigger pair combined inputs derived from B andcombinations of A, A' and' C, C'.

5. A multistage reversible digital counter comprising, a plurality ofstages, each stage as dened -in claim 2, and connections for utilizingas input pairs of any stage the output pairs of the next precedingstage.

6. A multistage reversible digital counter comprising The counter isturned onagain, the..

13 a plurality of stages, each stage as defined in claim 3, 2,479,802and connections for utilizing as input pairs of any stage 2,481,347 theoutput pairs of the next preceding stage. 2,516,146 7. A multistagereversible digital counter comprising 2,536,916 a plurality of stages,each stage as defined in claim 4, 5 2,539,623 and connections forutilizing as input pairs of any stage 2,656,106

the output pairs of the next preceding stage.

References Cited in the le of this patent UNITED STATES PATENTS 102,462,292 Snyder Feb. 22, 1949 14 Young Aug. 23, 1949 Riggeu Sept. 6,1949 Prugh July 25, 1950 Dickinson Jan. 2, 1951 Heising Jan. 30, 1951Stabler Oct. 20, 1953 OTHER REFERENCES Reversible Decade CountingCircuit, Regener, The Review of Scientic Instruments, vol. 17, No. 10,October 1946, pages 375-6.

